At present, electric devices have been made as small as possible in their device sizes, and an individual device has become approaching the smallest size limit thereof. In the case of, for example, CMOSs, which are current leading memory devices, it is expected that the minimum value of their channel length permitting their functions to be expressed would be 6 nm. In order to develop new techniques exceeding this limit, the development of new devices has been advanced on the basis of various ideas throughout the world.
For example, with respect to memory devices, two-terminal resistance switching devices have been researched, in which a large change in resistance is generated between on-states and off-states of the devices through the migration of atoms or a change in property of molecules. Typical examples thereof will be introduced hereinafter.
A technique introduced in the following Non-Patent Literature 1 is a technique of utilizing an electrochemical reaction between a silver sulfide electrode and a platinum electrode, to stretch and shrink silver particles, to control, through the silver atoms, the bridging and breaking between the electrodes, thereby realizing an atomic switch.
A technique introduced in Non-Patent Literature 2 is a technique of utilizing a redox reaction of catenane-series molecules and inducing the redox reaction of the molecules by a voltage, so as to open a channel, thereby realizing a switching device.
As described above, in recent years, reports have been made on switching devices utilizing the stretching and shrinking of a small number of metal atoms or a redox reaction of molecules.
As illustrated in FIG. 1, the inventors of the present invention proposed a two-terminal resistance switching device in which a voltage is applied to metallic electrodes with a nano-scale gap width across the electrodes (Patent Literature 1, Non-Patent Literature 3). The technique proposed in those literatures is a technique of applying a voltage to gold electrodes with a gap width of about 0.1 nm to 20 nm across the electrodes so as to control the gap width. It is demonstrated that, according to this technique, the resistance value of the gap portion can be controlled and the device can be applied as a non-volatile memory, utilizing the control of the gap width.
Further, the inventors of the present invention manufactured a gap electrode having an air gap width of 20 nm or smaller by using a carbon nanotube, to thereby try to manufacture a switching device in a region area that is far smaller than a limit wiring width (45 nm) of a current silicon process. It was confirmed that a switching phenomenon certainly does not occur in a case of using a simple single-layer carbon nanotube as a material of the electrode, whereas the switching phenomenon occurs with high probability in a case of using a single-layer carbon nanotube including fullerene molecules (C60) or carotene molecules as the material of the electrode.
However, in manufacturing a switching device by using the carbon nanotube including fullerene molecules or carotene molecules, it is necessary to perform a step of causing the carbon nanotube to include those molecules, which leads to a problem in terms of manufacturing efficiency. Moreover, some single-layer carbon nanotubes have a property of a semiconductor whereas other single-layer carbon nanotubes have a property of a metal, but it is extremely expensive to perform refinement for sorting out only the single-layer carbon nanotubes having the property of a metal, which are suitable for use as an electrode. Furthermore, there is also a problem that the single-layer carbon nanotube is vulnerable to heat and oxidation.
Patent Literature 1: Japanese Patent Application No. 2006-189380 (JP-A-2007-123828 (“JP-A” means unexamined published Japanese patent application))
Non-Patent Literature 1: Nature 433, (2005) 47-50.
Non-Patent Literature 2: SCIENCE 289, (2000) 1172-1175.
Non-Patent Literature 3: Nanotechnology 17, (2007) 5669-5674.